1. Field of the Invention
This invention relates generally to pressure sensor amplifier circuits and more particularly to circuitry which temperature compensates both itself and a semiconductor pressure sensor connected thereto.
2. Description of the Prior Art
Modern automobile engine control system presently require pressure responsive transducers and circuitry for engine management. More specifically, some such systems utilize manifold pressure sensors and amplifier circuits which provide an analog control signal which is a function of the engine manifold pressure. An analog-to-digital converter transforms the analog control signal into a digital control signal which is utilized by a microprocessor, for example, to control fuel injection.
Unfortunately, most semiconductor pressure transducers have undesirable temperature characteristics which are different for different transducers. More specifically, the sensitivity of the output signal to pressure change and the offset voltage of such devices undesirably vary with temperature change. The temperature coefficient of "span" or change in voltage output versus change in pressure typically has a negative temperature coefficient having a range of magnitude for different transducers. Offset voltage, which is the differential output voltage of the transducers at zero pressure, can have either a positive or a negative temperature coefficient each with a range of magnitudes for different transducers.
A prior art configuration for temperature compensating span utilizes a plurality of thermistors which vary the magnitude of the excitation voltage across the transducer to compensate for the undesirable changes in sensitivity with temperature. More specifically, a pressure transducer of one type can be considered as a bridge circuit and the thermistors are connected from each power input terminal of the bridge to a power supply line. The thermistors change the excitation voltage level so that the output voltage across the terminals of the bridge remain constant for a given change in pressure even though the temperature changes. Also, such thermistors have been shunted with temperature stable elements such as resistors to tailor the compensation characteristic. The combination of resistors, thermistors, and transducers has been adjusted by laser trimming through iterative operations over temperature to provide a composite device having a desired degree of temperature independence. These adjustments usually include sequential measurements over temperature and trimming. Such elements can also be trimmed to compensate for the undesired temperature dependence of the transducer offset voltage.
The above prior art techniques and configurations tend to be expensive because of the cost of discrete precision components and the complex trimming procedure. Moreover, even if the transducer and temperature compensating elements are trimmed to provide the desired temperature characteristics, the amplifier circuitry utilized with such compensated transducer configurations also tend to provide undesired temperature effects on the amplified analog output voltage.